Bloodstained Ritual of the Night Walnut

Obtaining the Walnut material will see you going toe-to-toe with the game's Chair Mimic enemies. You'll know you found them when you catch yourself fighting green chairs that also sport arms.

You'll find Chair Mimic in two of Bloodstained Ritual of the Night's areas. Specifically, look for them around Livre Ex Machina and the Forbidden Underground Waterway. The chance of them dropping a Walnut is 8% which is actually higher than that of quite a few other materials.